The present invention relates to a process for making a diffraction grating having a desired period and size in a desired location of a semiconductor substrate.
Integrated optics having semiconductor lasers, optical guides, optical modulators or optical demulti plexers formed monlithically on the semiconductor substrate are of interest because these devices are considered promising in the fields of optical communications and optical information processing.
Functional devices in integrated optics include optical multiplexers and demultiplexers which are important components for wavelength multiplexing, distributed feedback lasers, and deflecting mirrors. In order to fabricate these functional devices, the diffraction grating of a substrate in waveguide form is a particularly important basic element.
The development of technology for making a diffraction grating of a desired period and size in a desired location of a semiconductor substrate is indispensable to the commercial implementation of integrated optics.
Currently, there are two methods for fabricating the diffraction grating of a substrate in waveguide form, i.e. the holographic exposure method and electron-beam exposure.
In the electron-beam exposure method, a diffraction grating or other devices is formed on a semiconductor substrate by irradiation of a fine electron beam through a photoresist or oxide film. The scanning of the electron beam can be effected according to a computer-programmed pattern. By using the electron-beam exposure method, a diffraction grating of the desired size can be defined in the desired location of the substrate. However, the diameter of the electron beam is finite and cannot be made smaller than a certain limit (e.g. 0.1 .mu.m). It is therefore difficult to define a pattern with a very small period, although this is required for the diffraction grating to be formed in an optical integrated circuit. The lower limit of the period of the grating that can be formed by the current technology of holographic exposure is 0.5 .mu.m, which is not small enough to provide the microfine structure necessary for the diffraction grating in an optical integrated circuit.
A schematic diagram of the optics used in the holographic exposure method is illustrated in FIG. 1. A semiconductor substrate 1 has a photoresist or masking agent, such as silicon dioxide coating 2. A laser 3 such as a He-Cd laser emits a coherent light having a wave length of 4416 .ANG.. The laser light passing through a shutter 4 is reflected from a mirror 5 and split into two beams by a beam splitter 6. Each beam is expanded by beam expanders 7, 7. The respective expanded beams which now resemble a plane wave are reflected by mirrors 8, 8 and fall on the substrate 1 from opposite sides at an angle incidence angle .theta..
The beams, which are components of the coherent light form an interference pattern in a direction normal to the direction of its incidence. Upon exposure, these bands are recorded on the photoresist 2. By subsequent etching of the substrate 1, periodic ridges and grooves are formed on the substrate.
The period of the interference pattern formed by the holographic exposure method is given by: EQU d=(.lambda./2n.multidot.sin.theta.) (1)
wherein .lambda. is the wavelength of the laser light; n is the refractive index of the medium in contact with the resist; and .theta. is the angle at which the laser light falls on the substrate.
According to the holographic exposure method, a diffraction grating of an adequately small period can be made using a visible laser operating at shorter wavelengths, such as a He-Gd laser. The period of the grating is in the order of submicrons necessary for optical communication with visible light or near-infrared light. However, the need for causing two expanded beams to interfere with each other makes it difficult to form a diffraction grating in a very small limited area.
In order to provide a diffraction grating of a desired shape and size in a desired location, an attempt has been made to expose a limited area of the substrate through a slit or mask 9 placed on the photoresist 2 as shown in FIG. 2. However, the thickness of the slit or glass mask is not zero, and each beam of the laser light falling on the resist 2 through an aperture or transparent portion bends at the edge of the aperture or transparent area. As a result of this phenomenon (diffraction), the laser beam does not provide a regular plane wave but an irregular pattern of interference pattern, and a desired diffraction grating pattern is not obtained.